1. Field of the Invention
The present invention relates to a process for producing a superconducting article and more particularly, it relates to a novel process for producing a superconducting article which yields useful superconducting materials composed of compound oxides which exhibit higher critical temperatures of superconductivity such as Ba-Y-Cu type, Ba-La-Cu type, Sr-La-Cu type, Ca-Sr-Bi-Cu type or Tl-Ba-Ca-Cu type.
2. Description of the Related Art
In a superconducting condition, perfect diamagnetism is observed in a material and no difference in potential is observed and an electric current of a constant finite value is observed internally, and hence, a variety of applications of superconductivity have been proposed in a field of electric power transmission as a means for delivering electric power without loss.
Superconductivity can be utilized in the field of power electric applications such as MHD power generation, power transmission, electric power reservation or the like; in the field of transportation such as magnetic levitation trains or magnetically propelling ships; in high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like, in the medical field such as high-energy beam radiation unit; in the field of science such as NMR or high-energy physics; or in the field of fusion power generation.
In addition to the abovementioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a Josephson device which is an indispensable switching device for realizing a high-speed computer which consumes very reduced power.
However, the use of superconductors has been restricted because the phenomenon of superconductivity can be observed only at very low cryogenic temperatures. Among known superconducting materials, a group of materials having so-called A-15 structure show rather higher Tc (critical temperature of superconductivity) than others, but even the top record of Tc in the case of Nb.sub.3 Ge, which showed the highest Tc could not exceed 23.2 K. at most. This means that liquidized helium (boiling point of 4.2 K.) is the only cryogen suitable for use in realizing this very low temperature of Tc. However, helium is not only a limited costly resource but also requires a large-scaled system for liquefaction. Therefore, there had been a strong demand for other superconducting materials having higher Tc. No material which exceeded the abovementioned Tc had been found in ongoing studies over the past ten years.
It has been known that certain ceramic materials of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. These type of superconductors, however, possess a rather lower transition temperature of about 10 K. and hence usage of liquidized helium (boiling point of 4.2 K.) as a cryogen is indispensable to realize superconductivity.
The possibility of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986)189]
This new oxide type superconducting material is [La, Ba].sub.2 CuO.sub.4 or [La, Sr].sub.2 CuO.sub.4 known as the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxide. The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as 30 K. which is much higher than the known superconducting materials and hence it becomes possible to use liquidized hydrogen (b.p.=20.4 K.) or liquidized neon (b.p.=27.3 K.) as a cryogen which produces temperatures in the materials at which the materials exhibit superconductivity.
It was also reported in the U.S. newspapers that C. W. Chu et al discovered in the United States of America another type of superconducting material having the critical temperature of on the order of 90 K. in February 1987, and hence the possibility of the existence of high-temperature superconductors has burst on the scene.
However, the above mentioned new type superconducting materials have been studied and developed only in a form of sintered bodies as a bulk produced from powders but have not been shaped into a wire form. The reason is that the new type superconductors are ceramic materials of compound oxide which do not possess enough plasticity or process-ability in comparison with well-known metal type superconducting materials such as Ni-Ti alloy, and therefore they can not be shaped or deformed or are difficult to shape or deform into an elongated article such as a wire by conventional techniques such as wire-drawing technique (in which superconducting metal is drawn directly or in embedded condition in copper into a wire form).
Still, the above mentioned sintered ceramic materials must be shaped into an elongated structure when they are used as a superconducting wire in practice. However, the above mentioned superconducting materials obtained in a form of a sintered body are very fragile and tend to break or crack under even very weak mechanical stress. And hence, when they are shaped into a wire, special attention must be paid to their handling in order not to be broken.
A polycrystal having completely uniform crystal structure can not be obtained from particles having superconducting properties alone. Still, the phenomenon of superconductivity is apt to be easily broken in stronger magnetic fields and under the fluctuation or unhomogeneous distribution of temperature in the sintered body. Further, the abovementioned oxide type superconducting materials possess rather higher specific resistance and lower heat-conductivity above the critical temperature. Therefore, if the phenomenon of superconductivity breaks locally, the sintered body produces Joule heat caused by the superconducting current preserved therein and consequently explosive evaporation of cryogen is induced when the heated portion of the sintered body contacts with the cryogen. In order to avert this danger, in conventional metal type superconducting material, superconducting metal is shaped into a fine wire or filament a plurality of which are embedded in electroconductive metal to act as a by-pass of electric current when superconductivity break.
The oxide type superconducting materials are, however, difficult to be shaped or deformed into such filaments, because they do not have enough plasticity or processability in comparison with well-known metal type superconducting materials such as Ni-Ti alloy.
In order to realize a reliable and practical superconducting structure, it is indespensable that the structure possesses enough strength and tenacity which is sufficient to endure a bending force during usage and which also has as fine a cross sectional dimension as possible such manner that it can transmit current at higher critical current density and at higher critical temperature. However, conventional techniques generally can not produce wire shaped ceramic articles possessing satisfactory mechanical strength and tenacity as well as a higher dimensional ratio of length to cross section or produce such articles only with difficulty.
Taking the abovementioned situation into consideration, the present inventors have proposed processes for producing sintered ceramic wires having a practically usable higher dimensional ratio of length to cross section without using an organic binder which is a cause of deterioration of strength and tenacity. Such processes are disclosed in U.S. patent application Ser. No. 152,713 titled "Process for manufacturing a superconducting wire of compound oxide-type ceramic" filed in Feb. 5, 1988 and application Ser. No. 161,480 titled "Process for manufacturing a compound oxide-type superconducting wire" filed in Feb. 28, 1988 in which a metal pipe filled with material powder is subjected to plastic deformation such as by a wire-drawing technique by means of series of die and is then sintered. These solutions are themselves satisfactory but require mechanical working such as wire-drawing and hence can not be used with pre-shaped articles having relatively complicated configuration such as coils or rings.
Therefore, an object of the present invention is to overcome the abovementioned problems of the conventional technique and to provide a novel process for producing a superconducting article which has a higher Tc and higher stability as superconductor and which also has a higher degree of freedom in configuration without use any mechanical working such as wire-drawing.